Plastics are finding increasing use in manufactured goods. For example, certain automobiles have plastic body panels, and aircraft have plastic interior paneling and exterior skin panels formed of plastics and plastic composites. While plastics offer several excellent properties including light weight, formability, and low cost, plastics also have significant disadvantages. In general, plastic surfaces are not as hard or abrasion resistant as metal surfaces. Furthermore, while some plastics may be transparent, glass, which is much heavier and more expensive, remains the material of choice in certain critical applications such as safety glass in automobiles and in passenger aircraft windshields. Substituting polymeric materials such as stretched acrylic or polycarbonate would lead to lighter transparencies, but would also pave the way for re-designing the overall shape of cockpits, for example. A new class of transparent composites made of glass fiber reinforced polymer have been developed that have mechanical characteristics, such as tensile strength, which are comparable to aluminum. However, these transparent composites are soft and are highly susceptible to UV, chemical attack and or mechanically induced degradation. Currently, stretched acrylic materials are used to fabricate aircraft passenger windows. Acrylic is used because of its flexibility, light weight, and easy formability. However, acrylic is a soft material and hence can be easily scratched. Water absorption, chemical attack, and mechanically induced scratches can lead to crazing when stress is applied to acrylic materials, as in a passenger window application.
Industry wide, polymer based transparencies are protected against wear and other chemical/nature induced degradation through siloxane coatings. At the present time, polycarbonate and other types of polymeric windows are protected by sol-gel based polysiloxane coatings. The term sol-gel or solution-gelation refers to materials undergoing a series of hydrolization and condensation reactions. The sol-gel coatings are homogeneous mixtures of a solvent, an organosilane, an alkoxide and a catalyst that are processed to form a suitable coating. The sol-gel coatings provide high transmittance and limited durability against wear and UV induced degradation. Typically, a metal alkoxide or metal salt is hydrolyzed to form a metal hydroxide. The metal hydroxide then condenses in solution to form a hybrid organic/inorganic polymer. The ratio of organic to inorganic components in the polymer matrix is controlled to maximize the performance for a given application. For example, increasing the organic groups would improve flexibility but may compromise wear and environmentally induced durability. The sol-gel coating may include materials such as cerium or titanium to improve abrasion resistance and ultraviolet induced degradation of the coatings. A typical application process would consist of component surface cleaning, followed by the application of the coating via a flow, spray or dip process. The surface cleaning may be achieved by solvent wiping with, for example, isopropyl alcohol or exposing the component to oxygen plasma. The sol-gel coatings can be cured at room temperature or elevated temperatures. For example, stretched acrylics must be cured at temperatures less than 180° F.
Aircraft cockpit windows are currently made of multi-pane glass for strength and abrasion resistance. Efforts are on going to switch to polymeric material based flight deck windows as these materials are light and are amenable to forming desired shapes at a low cost. While plastics offer several excellent properties such as light weight, formability, and low cost, plastics also have significant short comings. In general, plastic surfaces are not as hard or abrasion resistant as glass or steel surfaces. Polymeric materials are susceptible to particle (e.g. sand)/water induced erosion and chemical crazing; protective hard coatings are needed to maintain the optical quality of the windows in use.
In addition, while a polymeric-glass laminate has weight savings over an all glass laminate, additional weight savings could be achieved by removal of the glass facing ply if erosion and abrasion were not a problem. The polymeric-glass laminate also suffers from thermally induced stresses which degrade service life due to the thermal expansion difference between the glass and polymeric layers. Matching contour between glass and polymeric plies poses manufacturing problems and often leads to optical and service related issues in the final part. Commercially available transparent hard coatings are in general solvent based polysiloxane. These coatings are applied through a dip, spray or float coat process and offer limited durability
Duplex coating schemes have been developed that offer improved performance. For example, a dual layer scheme has been developed that includes a relatively soft hard coat, such as polysiloxane with a harder but brittle top layer. Further developments to dual layer schemes include a multilayer scheme as well as a dual layer coating applied using only plasma deposition. While these newer coating developments offer outstanding durability, they transmit light over all wavelengths, including UV.
There is a need for a durable, transparent, hard coating that improves component lifetime by providing UV protection. The coating should provide improved resilience against chemicals commonly encountered in product maintenance, excellent weatherability characteristics, and UV protection. The coating should be both hard and flexible, so that it tolerates the flexing of the polymeric material due to operation and thermal stresses. The coating should be provided by a simple process and at a low cost.